1. Field of the Invention
The present invention relates to a control system for a steplessly variable power transmission.
2. Description of the Prior Art
There have been known a steplessly variable power transmission provided with a drive pulley, a driven pulley and an endless belt applied to V-shaped grooves formed on the drive and driven pulleys so that an engine power is transmitted to a wheel driving system with a given speed ratio (drive pulley rotation speed/driven pulley rotation speed) which can be continuously varied. The speed ratio in the steplessly variable power transmission is continuously varied in a manner that a hydraulic pressure which is controlled by solenoid valve is applied for control cylinders of the drive and driven pulleys to change widths of the grooves thus changing an effective diameter of the pulleys. As a result, a torque transmitted through the transmission is changed in accordance with the hydraulic pressure applied to the control cylinders of the pulleys.
In the steplessly variable power transmission, it is required to minimize the slip rate of the belt and prevent the damage thereof. Thus, it is necessary to keep a grip pressure applied to the belt by the pulleys at a proper value in response to the torque transmitted. For this purpose, it has been known that a solenoid valve for controlling a line pressure of a hydraulic circuit is controlled in accordance with engine speed, the speed ratio and the like so that the pressure applied to the belt through the pulleys is controlled to correspond to the torque transmitted therethough.
With this pressure control for the pulleys, it should however be noted that when some abnormalities occur in a control signal for the solenoid valve or in operation of the solenoid valve, the hydraulic pressure applied to the control cylinders for the pulleys could not be maintained at a proper value. In this case, the grip pressure for the pulleys would be unduly lowered to produce undesirable slip of the belt. In view of this, Japanese Patent Public Disclosure No. 62-52260, laid open to the public on Mar. 6, 1987, discloses a control system of the grip pressure for the belt in which when the line pressure of the hydraulic circuit is lowered below a predetermined value, the hydraulic pressure applied to the belt is controlled to a value corresponding to a spring force in the solenoid valve, or a minimum pressure, and a torque transmitted from the engine to the transmission through a magnetic clutch is controlled to a value smaller than a torque transmitted through the belt under a condition of the minimum hydraulic pressure with the maximum rotation ratio of the transmission (driven pulley rotation speed/drive pulley rotation speed). Namely, in this control, the engine torque transmitted to the drive pulley is controlled as low as possible to suppress the slip between the belt and the pulleys.
It should however be noted that in this control, a drivablility of vehicle may be deteriorated since the torque transmitted through the transmission is minimized.